A model for the action of hemoglobin has been developed in which the apparent cooperative effect is generated by the Bohr effect. When this is done, it becomes possible to explain oxygen binding, hydrogen ion binding, salt effects and aggregation effects in a unified manner. Studies are in process to test the model and to relate the model to the reactivity of the SH groups and the binding of 2,3-diphosphoglyceric acid (DPG) and other anions. In these studies the emphasis is being placed on an analysis of the energies of interaction under the various binding conditions. Oxidation of hemoglobin to methemoglobin is being studied under a variety of conditions with different reagents. Cooperative binding systems of importance in hormone action are under study.